Steam turbines of the modern type of construction for high efficiencies and high steam inlet temperatures comprise a rotor which is rotatable about an axis of rotation and which is surrounded by a casing. The casing is subdivided into an inner casing, which surrounds the rotor concentrically, and an outer casing, which surrounds the inner casing together with the rotor. Between the rotor and the inner casing is formed a steam duct which is in the form of an annular gap and through which the steam is conducted for the performance of work. The blading of the steam turbine is arranged in the steam duct and consists of alternately arranged rings of stationary guide vanes and of moving blades fastened on the rotor. The guide vanes are arranged on the inner wall of the inner casing, said inner wall delimiting the steam duct (see, for example, EP-A1-0 952 311 or U.S. Pat. No. 5,695,317 or U.S. Pat. No. B1-6,315,520).
Both the outer casing and the inner casing are conventionally horizontally divided thick-walled castings composed of a comparatively costly high-temperature alloy. For example, a steel casting is used for the outer casing. The inner casing, exposed to especially high pressures and temperatures, mostly consists of a special nickel-based alloy. Steam turbines with steam inlet temperatures of about 700° C. or above are currently in the planning stage. Pressures of several 100 bar, for example 350 bar, occur in this case.
According to current estimates, the starting time of steam turbines of the type described amounts to several hours, since, because of the large dimensions and wall thicknesses, the rotors and the casing require a long time before they can be heated to the operating temperature, without excessively high thermal stresses being generated. Further disadvantages of a high-mass inner casing are the high costs, since the nickel-based alloy is a very costly material. Also, for such large castings, there are long delivery times of several months. Since the inner casings are divided and the parts are screwed to one another via flanged connections, high-mass parting line flanges are present which make up a considerable proportion of the entire casing weight. Large and costly parting line screws for the flanged screw connection also have to be employed correspondingly.